1. Field of the Invention
The present invention relates to a multi-loop oscillator, and more particularly to a multi-loop oscillator which can control variation of an oscillating frequency of a ring oscillator according to variation of a supply voltage.
2. Description of the Prior Art
FIG. 1 is a circuit diagram of a conventional ring oscillator. As shown in FIG. 1, the ring oscillator includes an inverter I0, an even number of inverters I1 to I6, a plurality of capacitors C1 to C7, a NOR gate NOR, and an inverter I7. The inverter I0 receives an enable signal En to invert the received signal, and the inverters I1 to I6 are connected in series with each other. The plurality of capacitors C1 to C7 are connected between each input terminal of the inverters I1 to I6 and a ground, in order to adjust the RC delay of each inverter. The NOR gate NOR employs an input signal of the inverter I0 and an input signal of the inverter I6 as an input. The inverter I7 buffers and outputs an output of the NOR gate NOR.
The conventional ring oscillator constructed as mentioned above forms a single-loop inverter chain having odd number of terminals consisting of even number of inverters and NOR gate performing a function of an inverter.
The reason for forming the single-loop inverter chain is that an input signal passing through one loop is returned as a signal having a pole opposite to the input signal only when the input signal passes through an odd number of inverter terminals. That is, while the enable signal En maintains a high level state, when an input signal at a high level is inputted to the inverter I1, a signal at a low level is outputted from an output terminal of the NOR gate NOR after a predetermined time passes. Further, when a predetermined time passes again, a signal at a high level is outputted from the output terminal of the NOR gate NOR. These steps continue while the enable signal En maintains a high level state. Accordingly, an output signal “out” of the inverter I7 is a pulse signal having a predetermined oscillating frequency.
However, when a supply voltage reduces applied to each inverter constructing the ring oscillator, driving capability of each inverter deteriorates. Accordingly, a signal processing time in an each inverter increases, and this causes a period of the pulse signal outputted from the ring oscillator to increase. That is, the oscillating frequency of the ring oscillator reduces. This reduction of the oscillating frequency causes the charge pumping capability of a pumping circuit (not shown) using the ring oscillator to deteriorate.
In contrast, when the supply voltage increases, the oscillating frequency of the ring oscillator increases. This increase of the oscillating frequency causes the charge pumping capability of the pumping circuit (not shown) using the ring oscillator to increase excessively.
The supply voltage may be a low voltage or a high voltage according to exterior noise or environment. If the oscillating frequency of the ring oscillator is designed in match with low voltage condition, excessive charge pumping phenomenon may occur in most normal voltage states or a high voltage state. Especially, since an oscillation frequency is very shortened in the high voltage state, a pumping operation itself may not normally operate.